The invention relates to an operational amplifier (OA) particularly for signal amplification requiring wide bandwidth, high slew rate, and fast settling time.
Two parts can generally be distinguished in every OA: the input and the output stage. In the input stage the differential input signal of the OA is amplified and converted into a single signal. An additional amplification is accomplished in the output stage which also provides low output impedance, short circuit protection, etc.
Two transistors as voltage followers are used in the first stage of the OA in order to avoid the disadvantages of a differential amplifier, e.g. the high output impedance, or in that way enforced circuit concepts having just a moderate performance. The output currents of the voltage followers flow to a current mirror via a current follower each. The output of the input stage consists thus of a current source controlled by the differential input voltage of the OA. A good example of this common solution is the input stage of the very popular OA: uA 741.
A relatively high offset voltage is caused by an insufficient performance of the current mirror. The input impedance of the output stage must be high. The large number of the components and the specific operation mode of the individual transistors results in a slow signal amplification. This input stage is suitable only for monolithic integrated circuits.
The invention is intended to provide an OA having very low number of components and providing fast and high signal amplification. According to the invention an operational amplifier with two voltage followers and without differential amplifier in the first stage solves this problem, as outlined in claim 1.
The differential input signal is converted into the single signal in the easiest way. Furthermore, the common base as the operation mode of the transistors is favored in order to achieve a very high and fastest possible signal amplification. Another advantage is the output impedance of the input stage which can be very low.